How big could a black hole get?

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These cosmic sinkholes can swell to the mass of billions of suns, but something seems to stop them there…

There is no theoretical upper limit to the mass of a black hole. However, astronomers have noted that the ultra-massive black holes (UMBHs) found in the cores of some galaxies never seem to exceed about 10 billion solar masses. This is exactly what we’d expect from the rate at which we know black holes grow, given the time that’s elapsed since the Big Bang.

Furthermore, recent studies suggest that UMBHs cannot physically grow much beyond this anyway, since they would then begin to disrupt the accretion discs that feed them, choking the source of new material.

Do all spiral galaxies have black holes at their centre?

Black holes may provide a gravitational force that, in the words of Obi-Wan Kenobi, “binds the galaxy together”.

Although not certain, it seems very likely that most (if not all) spiral galaxies contain a supermassive black hole and sometimes more than one. That uncertainty exists for two reasons. First, we can’t physically examine every spiral galaxy in the Universe to be completely convinced of that fact. Second, theory tells us that it isn’t necessary for galaxies to contain central black holes – they will still hold together as galaxies without them.

For many galaxies, however, black holes (or rather their effects on their environments) are clearly observed in the turbulent core regions and the dynamics of their stars often indicate the presence of extremely massive objects.

There is also very strong evidence that black holes may be crucially important, perhaps even required, in the formation of galaxies in the early Universe. This would imply that indeed all types of galaxies (including spirals) contain a gravitational beast at their heart.